The present invention generally relates to improving mask fabrication by reducing defects on a mask. In particular, the present invention relates to using oxygen and an electron beam for minimizing and/or eliminating mask defects.
In the semiconductor industry, there is a continuing trend toward higher device densities. To achieve these high densities there has been and continues to be efforts toward scaling down the device dimensions on semiconductor wafers. In order to accomplish such high device packing density, smaller and smaller feature sizes are required. Since numerous conductive features are typically present on a semiconductor wafer, the trend toward higher device densities is a notable concern.
The requirement of small features (and close spacing between adjacent features) requires high resolution lithographic processes. In general, lithography refers to processes for pattern transfer between various media. It is a technique used for integrated circuit fabrication in which a silicon slice, the wafer, is coated uniformly with a radiation-sensitive film, the photoresist. The photoresist coated substrate is baked to evaporate any solvent in the photoresist composition and to fix the photoresist coating onto the substrate. The baked coated surface of the substrate is next subjected to selective radiation using a mask; that is, a mask is employed to effect an image-wise exposure to radiation. The mask permits radiation to contact certain areas of the photoresist and prevents radiation from contacting other areas of the photoresist. This selective radiation exposure causes a chemical transformation in the exposed areas of the photoresist coated surface. Types of radiation commonly used in microlithographic processes include visible light, ultraviolet (UV) light and electron beam radiant energy. After selective exposure, the photoresist coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist (depending upon whether a positive photoresist or a negative photoresist is utilized) resulting in a patterned or developed photoresist.
The mask is a critical element in lithography. Defects in the mask lead to imprecise exposure and consequent decreases in resolution, precise pattern formation and/or the quality of subsequent processing steps. For example, a contaminant particle on a mask may prevent radiation from contacting an area of a photoresist that should receive radiation, resulting in an incompletely exposed photoresist, which would lead to an undesirable pattern formation in the subsequently developed photoresist. Mal-formed structures inhibit the proper function of semiconductor devices. Contaminant particles, especially carbon-containing contaminant particles, may be present in the air and are often traced to the extensive use of photoresists that contain organic materials.
The scanning electron microscope (SEM) is an instrument commonly employed for the evaluation of surfaces in the semiconductor industry. The SEM forms an image by focusing an electron probe onto the surface of the specimen and the image contrast is formed using the secondary electrons or the high energy back-scattered electrons which are generated at or near to the surface. The SEM can provide data in connection with the surface topography including linewidth and critical dimensions.
However, when evaluating surfaces, the SEM tends to undesirably form a carbon-containing film and/or cause undesired electrostatic charges on a scanned surface. Often, the SEM leaves carbon-containing contamination in the form of a film over the scanned portion of the mask. An accumulation of electrostatic charge on the mask surface tends to attract the airborne contaminants. Due to the nature of mask processing and fabrication, mask surfaces are especially susceptible to contamination and accumulation of electrostatic charges.
The concern over contaminant particles is great because one defect or particle on a mask may constitute a fatal defect requiring expensive and burdensome mask replacement or reconstruction. Moreover, as geometries shrink, small defects have an increasingly detrimental impact on mask processing. Therefore, there is an unmet need for an efficient method for detecting and removing carbon contaminants from a mask during mask production.
The present invention provides an improved mask fabrication process and system using oxygen and an electron beam for minimizing and/or eliminating mask defects. The present invention particularly provides methods for reducing mask defects caused by carbon contamination using an oxygen plasma or gaseous oxygen combined with an electron beam flood exposure, wherein the reactive environment promotes the conversion of carbon-containing contaminants to carbon dioxide, thereby removing the mask defects from a mask. As a result of the present invention, a subsequently patterned resist of increased quality (fewer pattern defects, improved resolution, etc.) comparable to a patterned resist where the masks of the present invention are not employed is obtainable.
One aspect of the present invention relates to a method for reducing carbon contamination on a mask involving placing a mask plate having carbon-containing contaminants thereon in a processing chamber; simultaneously contacting the mask plate with oxygen and exposing the mask plate with a flood exposure of electron beams wherein the carbon-containing contaminants are converted to a by-product; and removing the by-product from the processing chamber.
Another aspect of the present invention relates to a method for in-line detection and reduction of carbon contamination on a mask involving placing a mask plate in a processing chamber; detecting for the presence of carbon-containing contaminants on the mask plate; if carbon-containing contaminants are detected, then simultaneously contacting the mask plate with oxygen and exposing the mask plate with a flood exposure of electron beams wherein the carbon-containing contaminants are converted to a byproduct; and removing the by-product from the processing chamber.
Yet another aspect of the present invention relates to an in-line system for detecting and reducing carbon contamination on a mask containing a detector for detecting carbon-containing contaminants on the mask plate; a controller coupled to the detector for determining whether the mask plate enters a processing chamber to remove carbon-containing contaminants; and the processing chamber for simultaneously contacting the mask plate with oxygen and exposing the mask plate with a flood exposure of electron beams.